CONICET | Buscador de Institutos y Recursos Humanos

Magnetic clouds (MCs) are objects in expansion during their travel through the heliosphere. In situ observations indicate that the MC front travels typically faster than its back, showing a clear empirical signature ofexpansion. With the aim of quantifying the expansion rate of MCs, we present here a statistical study of all the MCs observed in the inner heliosphere (from 0.3 to 1 AU) by the spacecraft Helios 1 and 2, during their complete period of operations. We first rotate the observed magnetic field into the MC frame (defined by the MC axis and the spacecraft trajectory). This allows a better localization of the MC boundaries for each event. We then split the sample in two subsets according to the characteristics of their velocity profiles, (a) those MCs with a significantly perturbed velocity profile due to the interaction with their surrounding solar wind (i.e. perturbed by streams), and (b) those that are not perturbed by them (not perturbed). We analyze the expansion velocity (defined with respect to the cloud center), Vexp and its dependence with several MC properties, as its size, its helio-distance, its translation velocity, etc. The removal of the correlations found lead to the definition of a dimensionless local expansion rate (ζ) for MCs. This definition of ζ is fully consistent with previous works, which defined the same quantity from theoretical considerations. We find significantly different values of ζ for perturbed and not perturbed events. Not perturbed MCs expand at rates consistent with the expected value from the global pressure decay in the surrounding solar wind (SW) for increasing helio-distances, while perturbed ones may present strong departures from that general rule. We interpret this departures for perturbed MCs as a consequence of their interaction with local SW streamson their expansion.